210 GENERAL ORNITHOLOGY part ii 



rest, to a variable number, being abortive, and melted into that 

 extraordinary affair called the rump-post or pygostyle (Gr. irvyrj, 

 puge, the rump ; o-tCAos, a post), which may consist of no fewer than 

 ten such metamorphosed tail-bones. It has usually a shape sug- 

 gesting the share of a plough (see Fig. 56, p«/), but is too variable 

 to be concisely described. The pygostyle supports the tail-feathers ; 

 and as these are morphologically one pair to each rectrix-bearing ' 

 vertebra, the number of tail-feathers may be primarily equal to the 

 number of vertebrae which fuse in the pygostyle. Thus the swan 

 is said to have ten vertebrae in this mass ; a wild swan (Cygnus 

 colvmbianus) has twenty tail-feathers. In this view, six should he 

 the usual composition of the share-bone. A bird's tail is really 

 more extensive and lizard-like than commonly supposed ; thus the 

 swan, besides its ten in the pygostyle, has seven free caudals, and' 

 ten urosacrals — twenty-seven post-sacral vertebrae in all (Huxley). 

 In the raven, the free caudals are six, exclusive of the pygostyle. 

 These all have large flaring transverse processes and moderate 

 spinous processes, and the latter ones are also provided with hypa- 

 pophyses, some of which are bifurcate. The pygostyle in many 

 birds expands below into a large circular or polygonal disk. 



2. THE THORAX: EIBS AND STERNUM 



The Thorax (Gr. ddpa^, a coat of mail ; in anatomy, the chest ; 

 adj. thoracic ; see Fig. 56) is the bony box formed by the ribs on each 

 side, the breast-bone below, and the back-bone above. In birds it 

 is very extensive, including most or all of the abdominal as well as 

 the thoracic viscera, and its cavity is not partitioned off from that 

 of the belly by a completed diaphragm, though a rudimentary struc- 

 ture of that kind is found in the Apteryx. The thorax is usually 

 soldered behind to the pelvis by union of one or more pairs of ribs 

 with the ilia ; in front it always and entirely bears the pectoral arch 

 (see p. 215). The thorax is very movable in birds, by reason of 

 the great length and jointedness of the ribs. 



The Ribs (Lat. costa, a rib ; pi. costx; adj. costal; see Fig. 56, 

 c, c', B, cr, sr, u), as said above, are the pleurapophysial elements of 

 vertebrae, which remain small and ankylosed, or become long and 

 free. In the latter state only are they " ribs " in ordinary language. 

 The one or more cervical ribs, however elongated, and the abortive 

 lumbar and urosacral ribs, are to be excluded from the present 

 description, and have been already considered. True ribs are those 

 which belong to the dorsal vertebrae proper, and are jointed in 

 themselves ; that is, have articulated hcumapophyses (see p. 203), by 

 which they may or do articulate with the sternum. Such true ribs 

 are fixed, when they reach from back-bone to breast-bone ; fioating, 



